Not able to select dropdown in UIWebView for IOS 9

I am display a website's page in UIWebView which is having a dropdown. User is able to select values from dropdown but when application is closed and again opened then this dropdown stops working means user is not able to select it.

Strange thing is that if we display alert box of javascript onclick on that dropdwon then everything works fine after tapping on ok button of alert box (means dropdwon becomes clickable) , so what can be a reason and solution for it?